Field of the Invention
Embodiments of the invention generally relate to methods to control lost circulation in a lost circulation zone in a wellbore during continuous drilling with a drilling mud. More specifically, embodiments of the invention relate to methods for converting a drilling mud into a gel-based LCM (lost control material) composition during continuous drilling.
Description of the Related Art
Lost circulation is one of the frequent challenges encountered during drilling operations. Lost circulation, which can be encountered during any stage of operations, occurs when drilling fluid (or drilling mud) pumped into a well returns partially or does not return to the surface. While some fluid loss is expected, fluid loss beyond acceptable norms is not desirable from a technical, an economical, or an environmental point of view. About 75% of the wells drilled per year encounter lost circulation problems to some extent. Lost circulation is associated with problems with well control, borehole instability, pipe sticking, unsuccessful production tests, poor hydrocarbon production after well completion, and formation damage due to plugging of pores and pore throats by mud particles. In extreme cases, lost circulation problems may force abandonment of a well. In addition, delays in controlling lost circulation can lead to highly complex problems, including the failure to control the lost circulation in any meaningful way.
Billions of dollars are lost per year due to lost circulation in drilling operations. Lost dollars are due to losses of drilling fluids, losses of production, and the costs of lost circulation materials (LCMs) used in combating lost circulation.
Lost circulation can cause environmental problems if drilling muds or LCMs interact with the environment surrounding the reservoir. Conventional LCMs pose a risk to sensitive environments, such as marine environments because they are not biodegradable and can be toxic to marine life. Public awareness of drilling operations, including the drilling fluids used, has contributed to demands from environmental regulatory bodies to develop biodegradable and virtually non-toxic LCMs.
Lost circulation can be categorized as seepage type, moderate type, severe type, and total loss, referring to the amount of fluid or mud lost. The extent of the fluid loss and the ability to control the lost circulation with an LCM depends on the type of formation in which the lost circulation occurs. Formations with low permeability zones, i.e., those with microscopic cracks and fissures, usually have seepage type lost circulation. Seepage type lost circulation experiences a loss of less than 10 bbl/hour for water based drilling muds, or about 10 bbl/hr for oil based drilling muds. Formations with narrow fracture sizes and lower fracture density usually trigger a moderate loss of drilling mud. A moderate type lost circulation experiences a loss at a rate in the range of about 10 bbl/hr to about 100 bbl/hr. Formations with high permeability zones, such as super-K formations, highly fractured formations with large fracture sizes and high fracture density, often experience very high mud loss with a drastic increase in total mud and mud management costs. A severe type lost circulation experiences losses of greater than about 100 bbl/hr. Formations with inter-connected vugular and cavernous zones or formations with induced inter-vugular connection often cause massive loss of drilling mud with no return of circulation. It is possible for one wellbore to experience all of these zones.
Other formations may experience lost circulation if an improper mud weight is used while drilling. Such formations include narrow mud weight window, low fracture gradient, depleted reservoir pressure, formations with soluble minerals such as halite, evaporate, and anhydrite.
In general, seepage type and moderate type losses occur more frequently than severe type lost circulation. In the Saudi Arabian fields, however, the formations encountered while drilling reservoir and non-reservoir sections have unique depositional histories and matrix characteristics that make the super-K, fractured, vuggy, cavernous, faulted characteristics of the carbonate rock formations prone to moderate to massive loss of drilling fluid. Some of the losses are so massive that hundreds of barrels of mud are lost in an hour with no return of fluid to the mud return line, as the rate of loss usually exceeds the rate of replacement of drilling mud. Thus, even though the frequency of severe lost circulation is less than seepage or moderate lost circulation, severe lost circulation has a significant economic impact on drilling operations.
LCMs are used to mitigate the lost circulation by blocking the path of the fluid. The type of LCM used in a loss circulation situation depends on the extent of lost circulation and the type of formation. Conventional LCMs, currently available in the industry, include particulates, flaky materials, granular materials, and gel LCMs including cross-linked gels, cross-linked polyacrylamrides, polyacrylates, super absorbing polymers (SAP), or a combination of the above. Conventional gel LCMs typically contain one or more polymers, one or more monomers, one or more cross-linkers, including chemical cross-linkers, a cross-linking initiator, and a fluid phase, such as water or oil. Some formulations may include particles.
For zones experiencing seepage type to moderate type lost circulation, conventional LCMs that include particulates, flakes, gels and/or a combination are often effective in controlling the loss zones. Polymeric and gel LCMs are also commonly used to control moderate to severe loss of circulation, due to their ability to swell, gel, crosslink, and/or expand. For example, SAPs expand many times in volume in the presence of water. The swelling, gelling, crosslinking, and/or expansion of the LCMs helps to stop the loss of drilling mud by plugging the fractures and/or the vugs. However, many high permeability zones experience limited success in attempts to control a lost circulation event, even with the use of conventional non-gel and gel LCMs. For formations with massive loss of drilling mud, current chemical methods of loss control rarely work.
Poor control in a lost circulation zone is often due to the LCM itself. The efficacy of a gel LCM depends in large part on the fracture dimensions, but also on the gel characteristics, namely the gel stiffness modulus and the yield strength. The gel stiffness modulus and the yield strength are indicative of the extent to which the gel LCM resists flow when forces are applied. Gel stiffness modulus is the extent to which a material resists deformation in response to an applied load, i.e. it is a measure of the rigidity of the material. Yield strength is a measure of the strength of a material, it is the force required to initiate plastic deformation. A high gel stiffness modulus and high yield strength indicate a gel that is resistant to deformation and that is therefore likely to solidify into a rigid gel. A gel with a low yield strength and low gel stiffness modulus is likely to form a soft gel system. A soft gel can control seepage type loss zones, but because soft gels cannot resist the stresses caused by fluids being pumped into the formation, a soft gel LCM will continue to move through the fractures and channels of moderate to severe loss zones without creating an effective flow barrier. If the gel LCM cannot seal the lost circulation zone effectively, it may not bring the mud loss below the maximum allowable limit. In some cases, the gel may not be capable of solidifying at all. Tests indicate that especially in vugular formations, conventional gel LCMs perform poorly.
Conventional gel LCMs usually have poor thermal stability, chemical stability, low gel stiffness modulus, low yield strength, and low tolerance for salt, making them unsuitable for some environments, e.g., marine, and thus have limited capacity in controlling loss of circulation, especially in highly fractured and cavernous formations.
In addition, the formulations of conventional gel LCMs require special preparation and handling. Special preparations can include the order in which the components are mixed, mixing techniques, or the need for specialized mixing units. If the formulation guidelines are not followed precisely, the conventional gel LCM may not obtain homogeneous gel characteristics. Careful handling implies the placement and pumping of the LCM into the formation. Conventional gel LCMs require precise placement in the formation due to the reaction kinetics of the polymers and cross-linkers. Proper placement ensures that the materials reach the proper gel characteristics at the target location. Proper placement in turn depends on the pumping schedule and the pumping units, which often must be highly specialized. In addition, drilling operations are usually stopped until the lost circulation zone is sealed and fluid losses to the formation are reduced to an acceptable level.
The requirements for preparation and placement mean that significant time can lapse between reaching lost circulation and beginning control measures with conventional gel-based LCMs. At a minimum, the time lapse translates to a substantial volume loss of drilling fluid. At worst, the extended preparation time may aggravate the problem, turning a manageable lost circulation problem into a situation in which lost circulation control is not possible and the entire well must be shut-down.
The industry needs an alternative lost circulation treatment that can be prepared quickly to control moderate to high mud losses. Beginning a lost control treatment process as soon as possible after the loss zone is encountered is desirable. A suitable alternative that overcomes the drawbacks of conventional gel LCMs to combat lost circulation and avoid the operational complexities associated with delayed lost circulation treatment is desirable.
A gel-based LCM that shows improved yield strength and gel stiffness modulus, and thus effectively mitigates mud loss, reduces volume of LCM pill, and meets environmental regulations is desired.